High performance level in terms of catalyst activity and stereospecificity has been obtained with the catalysts described in the European patent No. 045977. Said catalysts have, as a solid catalysts component, a magnesium halide on which is supported a titanium halide, preferably TiCl4, and an electron-donor compound, selected from specific classes of carboxylic acid esters, and, as co-catalyst component, a system formed of an Al-trialkyl compound and a silicon compound containing at least one Si—OR bond where R is a hydrocarbyl radical.
Most of the polypropylene catalysts used nowadays are based on phthalate diesters as internal donors used in combination with an aluminium alkyl cocatalyst and with alkylalkoxysilanes as selectivity control agents (external donors). A particular type of these catalysts, those endowed with a relatively high porosity are also used in the preparation of high impact heterophasic copolymers comprising a crystalline propylene homo or copolymer matrix and a substantial amount, in certain applications more than 50% wt, of a very low crystalline, highly soluble in xylene ethylene based copolymer.
For this last specific application the catalyst has to show also a good morphological stability allowing to produce a substantial amount of low crystallinity copolymer without showing phenomena of clogging, formation of chunks and particle agglomeration or adhesion to reactor walls.
The high porosity phthalate-based catalysts are generally endowed with this feature as shown by EP-A-395083 disclosing obtained by contacting a titanium compound, an electron donor compound (phthalate), with a MgCl2.(EtOH)m in which m is from 0.15 to 1.7 that has been in turn obtained by partial dealcoholation of adducts with a higher alcohol content.
However, the fact that sometimes the high impact compositions produced with this catalyst show a too high content of oligomers that negatively affect their properties and the toxicity problems that might be associated with the use of phthalates leads to evaluate alternatives for their replacement.
The European patent No. 0361494, EP728769 and the international patent application WO99/57160 describe very active solid catalyst components for the polymerization of olefins comprising, as an internal electron-donor compound, a 1,3-diether characterized by a specific structure and/or by specific reactivity characteristics toward the anhydrous magnesium chloride and TiCl4. In WO99/57160 an additional donor selected from esters of carboxylic acids and in particular malonates, can be present.
The catalysts obtained from the reaction of said catalyst components with an Al-alkyl compound exhibits a so high activity and stereospecificity in the polymerization of olefins, that the use of an external electron-donor can be avoided. It has to be noted that in the above mentioned documents the 1,3-diether based catalysts are always prepared starting from a support which does not generate a high porosity catalyst. When trying to apply the approach described in EP 395083, using 1,3-diether as internal donor, it has been observed that the rate of incorporation of the diether in the catalyst is much lower and the catalysts obtained show performances worsened to an unacceptable level in terms of activity/stereospecificity balance. It would be therefore important to find a way of generating a catalyst free from phthalates having a good morphological stability and high activities even when a 1,3-diether is used as internal donor.